Root cannot start X applications when su'ing from a user's X session. Using 'xhost +' or similar fixes it but this is not an ideal or intuitive solution. Please accept my apologies if this is an intentional security feature, but if 'rm -rf' is OK I really don't mind the odd unauthorized X app.
gentoo tries to install everything with minimal security allowances, and this is no exception ... xfree really shouldnt allow anything unless the user wants it to ... i for one prefer the 'disabled by default' approach gentoo takes to everything
do not use 'xhost +', but rather put 'xhost local:root' in the users's .xinitrc or .xsession if he wants root to have access to his Xsession by default.